Intravenous (IV) Delt 9-tetrahydrocannabinol (THC) induces transient psychotic symptoms in healthy subjects and in schizophrenic patients, but the
psychotomimetic mechanism is unknown. One possibility is that THC stimulates dopamine (DA) release in the striatum. In this study we tested whether
IV THC led to an increase in striatal DA release compared to placebo. We also investigated whether DA release and positive psychotic symptoms were
related. Eleven healthy male volunteers completed two 123I-iodobenzamide ([123I]IBZM) single photon emission tomography (SPET) sessions and
received IV THC (2.5 mg) or placebo in a randomized counterbalanced order, under double-blind conditions. Analysable data were obtained from nine
participants. The Positive and Negative Syndrome Scale (PANSS) was used to rate psychotomimetic effects. Striatal binding index values were calculated
using the occipital cortex as a reference region. Both the PANSS positive and general symptoms increased significantly at 30 min following IV THC.
There were no significant differences in binding index in the caudate or putamen under THC compared to placebo conditions. Positive psychotic
symptoms and DA release were unrelated. THC did not lead to a significant increase in DA release even though the dose was sufficient for participants to
have psychotic symptoms. These findings do not support a central role for striatal DA in THC-elicited psychosis.